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Alluvial Deposits

The dynamics of the movement of sedimentary particles in streams and rivers are of interest in the formation of placers. The factors involved are many, including the velocity of the water, which depends essentially on the gradient; the degree of turbulent flow, the specific gravity of the minerals being transported and the nature of the stream or riverbed.

Two types of solid load transported by streams and rivers can be distinguished - the suspended load and the bed load. The suspended load is maintained in the body of water mainly by turbulence, although flotation of sedimentary particles attached to air bubbles may be important in places. The bed load consists of material that is rolled, pulled, slid or otherwise swept along the bed of the stream by traction or saltation (skipping of pebbles along the bottom in a series of leaps). In addition to these movements there is a constant creep of the sediment down gradient. This effect is small where the slope is minor, but may be marked where high gradients prevail. As an approximation it is said that the transporting power of a stream varies as the sixth power of the velocity. Calculations show that a stream running 1 m/sec will carry a stone or nugget weighing about 3 oz; at 2 m/sec one weighing 45 lb; at 5 m/sec one weighing 1.5 tons; and so on. It is easy to understand how a spate of water moving down a gulch at 10 m/sec, as during flood times, can move boulders weighing 100 tons and more.

The evolution of a landscape following a general uplift above sea level follows definite patterns depending essentially on the amount of rainfall received. In districts with humid climates most rivers flow into the oceans, although interrupted in places by lakes. Four normal stages of evolution of drainage systems in humid areas can be recognized:
(1) initial
(2) youthful
(3) mature
(4) old age

During the initial stage the gradients are low, depressions become filled with water, and streams lazily follow various gulleys and flaws in the rock. The rate of erosion is minimal, and few if any placers are formed. As the youthful stage sets in an integrated pattern of major streams with numerous tributaries develops by downward and headward up-gradient erosion, most lakes disappear and V-shaped valleys, and in places steep gulches, canyons and gorges, mark the landscape. With time the normal evolutionary pattern develops further to maturity. Gulch, creek, stream and some river placers are characteristically developed during these stages. As the rivers erode down toward their base level, lateral erosion develops wider and wider valleys with gently sloping sides, flood plains are formed, and meandering courses that wander hither and thither from channel to channel are characteristic. Some creek and stream placers continue to develop, but river, flood plain and deltaic placers are the mark of this stage. Continued slow downcutting of rivers and streams or uplift may leave the remnants of earlier valley floors as terraces in which remnants of the river flood plain placers are preserved. In old age the level of the whole land is reduced to a peneplain characterized by a low relief with low rounded hills, shallow valleys, rolling landscapes and sluggish meandering rivers, marked by oxbow lakes that flow in various directions following the very low gradients that prevail. Few if any placers are formed during this stage.

In arid climates the evolutionary pattern is significantly different, mainly because of reduced rainfall, the absence of thick soil cover and the lack of vegetation. Wind erosion is more important in such climates; extensive downhill creep along valley sides is reduced because of the lack of moisture, and numerous basins of interior drainage develop generally with no connections to oceans. The infrequent precipitation during the youthful stage gives rise to numerous nonintegrated streams in steep-walled valleys that cut deeply back into the highlands. With increasing maturity a poorly integrated drainage develops mostly into closed basins that are marked by pediments on which alluvial fans may form. In old age the drainage pattern is generally completely disintegrated, there being in most districts no streams of any length and commonly no interconnection between those that do exist during a pluvial period. Arid conditions are not particularly favorable for the formation of placers, although some may form in gulches, in canyons and in alluvial fans on pediments where the conditions are right. Arid areas are, however, often characterized by great spates and walls of water that come off the highlands or mountains during cloud bursts, rush down the dry washes, gulches, canyons and valleys carrying everything before them, and finally debouch onto the alluvial fans on pediments or into the closed basins. Such violent activity is not conducive to the formation of placers; on the contrary any that formed would be sluiced down the gulches, creeks and canyons and the gold spread indiscriminately over the lower valley bottoms and inland basins.

Desert placers as a whole are small and the pay streaks are often erratic and commonly scattered. 'Bajada' or 'pediment placers' in desert terrains may be more productive, but they are commonly difficult to work.

The above descriptions characterize the normal evolution of drainage patterns and the formation of placers under the extremes of humid and arid conditions. But many abnormal conditions prevail in any particular district. Sudden uplifts at the mature or old age stages cause renewed downcutting of channels and destruction or deep burial of placers already formed. Similarly, great or sudden increases in rainfall may produce floods that sluice out the gulches, creeks and canyons, change the courses of the rivers, and greatly modify, destroy or bury the most recent placers. Depressions of valleys may result in basins in which lakes develop or inlets are formed, which are invaded by the ocean. These ultimately become filled with sediment burying deeply any placers formed in the valleys. Finally, glaciers forming in high mountains and extending down gulches and valleys may plough out all sediments and scatter the placer materials far and wide.

The location of the pay streaks in placers is of prime interest to the placer miner. Given an adequate primary gold source pay streaks in general are fairly uniform and have considerable continuity in moderately hilly country where uniform rainfall has prevailed and where deep secular decay has been followed by a gradual restricted uplift. Any aberrations in this ideal pattern invariably cause marked variations in the tenor and continuity of the pay streaks. The richest pay streaks are those produced by reworking of preexisting auriferous gravels.

The law of the pay streak in placer deposits is variable depending on whether the placer is formed in gulches, in river channels, on flood plains or in deltaic deposits. Lindgren (1911, p.66) said of the pay streak referring to the deposits of the Sierra Nevada of California, some of which are buried beneath Tertiary lava flows:

"It has become almost an axiom among miners that the gold is concentrated on the bedrock and all efforts in placer mining are generally directed toward finding the bedrock in order to pursue mining operations there. It is well known to all drift miners, however, that the gold is not equally distributed on the bedrock in the channels. The richest part forms a streak of irregular width referred to in the English colonies as the "run of gold" and in the United States as the "pay streak" or "pay lead". This does not always occupy the deepest depression in the channel and sometimes winds irregularly from one side to the other. It often happens that the values rapidly diminish at the outside of the pay lead, but again the transition to poorer gravel may be gradual. An exact explanation of the eccentricities of the pay lead may be very difficult to furnish. Its course depends evidently on the prevailing conditions as to velocity of current and quantity of material at the time of concentration."

The features of the evolution of valleys are discussed above but this knowledge is only of general value in studying placers. To accurately determine the location of their pay streaks, especially in the buried types and those on flood plains and deltas, requires detailed profiling by trenching or overburden drilling. Geophysical methods may assist in some terrains. Where the cover is light and the bedrock magnetically low, a magnetometer survey may outline the position of the magnetic 'black sands' so commonly accumulated with gold.

[ Placer Deposits 1  2  3  Eluvial  5  6  Alluvial  8  9  10  Examples  12  13  14  15 ]

Maps of alluvial gold deposits in: California, Western Canada, Eastern Canada, Russia, World
Maps of primary gold deposits in: Precambrian, Paleozoic, Mesozoic, Cenozoic Rocks

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011